The present invention relates in general to electro-mechanical devices, and in particular to linear motion actuators.
Soleniods are traditionally used to actuate mechanisms by the application of a voltage to an electromagnetic coil. Solenoids are expensive and require considerable design effort to ensure that the mechanical load requirements are consistent with the available force profile of the solenoid. This can be particularly challenging since the solenoid provides less force near the beginning of its stroke and provides exponentially more force as the stroke reaches the end of its travel. Solenoids suffer reliability problems because magnetic flux must bridge the plunger""s sliding bearing and a residual magnetic force of close tolerance must prevent the plunger from magnetically sticking to the pole face. If either of these design parameters becomes too marginal, the solenoid performance is radically altered.
Linear actuators have been designed where a motor drives a threaded shaft and a corresponding threadedly coupled nut. The nut translates laterally when prevented from rotating by a guiding surface. The motor may be driven in one direction to emulate the drive stroke of a solenoid and driven in the other direction to return the nut and an attached actuator means to a home position. To define the stroke of the motor driven linear actuator, axial stops have been used which may generate thrust loads in the extended or retracted position. To eliminate driving the motor in both directions, an axial thrust spring has been used with the appropriate thread design to enable the thrust spring to rotate the shaft and translate the nut to a home position. Axial loads may cause motor damage or the threads to bind and thrust return springs, besides causing axial loads, put severe restrictions on the design of the threads to allow a non powered return of an extended actuator.
There is, therefore, a need to have an electrically driven linear actuator device that has neither a sliding bearing nor a requirement for a magnetic residual. There is also a need for a linear actuator that eliminates thrust loads on a drive motor which may bind threads of a linear actuator or reduce motor life. Furthermore, there is a need for the linear actuator device that generates force relatively independent of travel position.
A low-cost direct current (DC) motor adapted with a threaded shaft is mounted into a reference frame which keeps the DC motor case and thus the stator fixed while the threaded shaft rotates. The threaded shaft is secured to the inner portion of a torsional spring. The outer portion of the torsional spring is secured to the reference frame. A translation actuator is threaded onto the threaded shaft and incorporates a first and a second actuator stop. The threaded shaft has a circular raised portion incorporating a first and a second shaft stop surface. The first actuator stop engages the first shaft stop surface which prevents the translation actuator and the threaded shaft from binding when in a retracted position. The second actuator stop engages the second shaft stop surface and prevents the translation actuator and the threaded shaft from binding when in an extended position. An extension of the translation actuator is operable to contact a mechanical load. The first and second actuator stops define the travel of the translation actuator. The reference frame is adapted with an engaging section that contacts a guide portion of the translation actuator to prevent the translation actuator rotation as the threaded shaft is rotated. The engaging section also guides the translation actuator as it linearly moves. Current supplied to the DC motor windings generates torque, dependent only on current amplitude and the DC motor torque constant. The DC motor torque turns the threaded shaft, loads the torsion spring, and drives the translation actuator which linearly translates and moves the mechanical load. When the current to the DC motor is removed, energy stored in the torsion spring rotates the threaded shaft in the reverse direction until the first actuator stop again engages the first shaft stop surface and prevents the translation actuator and shaft threads from binding.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.